/etc/network/interfaces: deal with OVS allow- lines

/etc/network/interfaces: deal with OVS allow- lines

* __read_etc_interfaces:
Delete OVS ports from "allow-$OVS_BRIDGE" option lines in
order to prevent them from being duplicated or kept after
removing the port from the bridge.

* __write_etc_interfaces:
Deleting unused OVSPorts has the side effect of them not
being written out at all. If, however, they are
physical interfaces they'll reappear the next time the
configuration is read, because they're added from
/proc/net/dev.
Fix: if the deleted interface matches the same condition as
in read_etc_interfaces, readd it with the standard options:
{exists => 1, method => manual }
This is a purely aesthetical change in order to make sure a
write()->read()->write() chain doesn't produce two
different files each write().